The mRNA of the lipogenic malic enzyme in FRTL-5 cell line is post-trancriptionally regulated by the thyroid stimulating hormone. The malic enzyme mRNA level increases about 5 fold within 6 hours after thyroid stimulating hormone addition and stays at this level for 18 hours there after. This increase is followed by a gradual decline, reaching the basal level at 72 hours after addition of the hormone. The re-addition of this hormone did not prevent the decrease. The treatment of the cells with the hormone and cyclohexamide did not alter the level of the malic enzyme mRNA, contrary to the treatment with actinomycin D. Actinomycin D added 23 hours after the hormone addition abolished the decrease in the malic enzyme mRNA level for 48 hours. Thus, it appears that ongoing protein synthesis is required to alter the stability and degradation rate of this message. Experiments are underway to establish if these changes are a cell cycle related. Selection of the malic enzyme-specific intronic probes was performed in order to investigate nuclear stabilization of the rat liver malic enzyme transcript by thyroid hormone. Among 3 intronic probes tested so far, one probe hybridizes with the malic enzyme RNA only, and shows 11-fold stimulation by hormone. The increase is in agreement with that of cytoplasmic mRNA. Two such intronic probes are under search. Northern blot analyses of nuclear transcripts and kinetic studies will be performed with these specific probes.